JSON is a subset of YAML 1.2. The JSON produced by
this module’s default settings (in particular, the default separators
value) is also a subset of YAML 1.0 and 1.1. This module can thus also be
used as a YAML serializer.

If ensure_ascii is true (the default), all non-ASCII characters in the
output are escaped with \uXXXX sequences, and the result is a
str instance consisting of ASCII characters only. If
ensure_ascii is false, some chunks written to fp may be
unicode instances. This usually happens because the input contains
unicode strings or the encoding parameter is used. Unless fp.write()
explicitly understands unicode (as in codecs.getwriter())
this is likely to cause an error.

If check_circular is false (default: True), then the circular
reference check for container types will be skipped and a circular reference
will result in an OverflowError (or worse).

If allow_nan is false (default: True), then it will be a
ValueError to serialize out of range float values (nan,
inf, -inf) in strict compliance of the JSON specification.
If allow_nan is true, their JavaScript equivalents (NaN,
Infinity, -Infinity) will be used.

If indent is a non-negative integer, then JSON array elements and object
members will be pretty-printed with that indent level. An indent level of 0,
or negative, will only insert newlines. None (the default) selects the
most compact representation.

Note

Since the default item separator is ',', the output might include
trailing whitespace when indent is specified. You can use
separators=(',',':') to avoid this.

If specified, separators should be an (item_separator,key_separator)
tuple. By default, (',',':') are used. To get the most compact JSON
representation, you should specify (',',':') to eliminate whitespace.

encoding is the character encoding for str instances, default is UTF-8.

If specified, default should be a function that gets called for objects that
can’t otherwise be serialized. It should return a JSON encodable version of
the object or raise a TypeError. If not specified, TypeError
is raised.

If sort_keys is true (default: False), then the output of
dictionaries will be sorted by key.

To use a custom JSONEncoder subclass (e.g. one that overrides the
default() method to serialize additional types), specify it with the
cls kwarg; otherwise JSONEncoder is used.

Note

Unlike pickle and marshal, JSON is not a framed protocol so
trying to serialize more objects with repeated calls to dump() and
the same fp will result in an invalid JSON file.

Keys in key/value pairs of JSON are always of the type str. When
a dictionary is converted into JSON, all the keys of the dictionary are
coerced to strings. As a result of this, if a dictionary is converted
into JSON and then back into a dictionary, the dictionary may not equal
the original one. That is, loads(dumps(x))!=x if x has non-string
keys.

If the contents of fp are encoded with an ASCII based encoding other than
UTF-8 (e.g. latin-1), then an appropriate encoding name must be specified.
Encodings that are not ASCII based (such as UCS-2) are not allowed, and
should be wrapped with codecs.getreader(encoding)(fp), or simply decoded
to a unicode object and passed to loads().

object_hook is an optional function that will be called with the result of
any object literal decoded (a dict). The return value of
object_hook will be used instead of the dict. This feature can be used
to implement custom decoders (e.g. JSON-RPC
class hinting).

object_pairs_hook is an optional function that will be called with the
result of any object literal decoded with an ordered list of pairs. The
return value of object_pairs_hook will be used instead of the
dict. This feature can be used to implement custom decoders that
rely on the order that the key and value pairs are decoded (for example,
collections.OrderedDict() will remember the order of insertion). If
object_hook is also defined, the object_pairs_hook takes priority.

Changed in version 2.7: Added support for object_pairs_hook.

parse_float, if specified, will be called with the string of every JSON
float to be decoded. By default, this is equivalent to float(num_str).
This can be used to use another datatype or parser for JSON floats
(e.g. decimal.Decimal).

parse_int, if specified, will be called with the string of every JSON int
to be decoded. By default, this is equivalent to int(num_str). This can
be used to use another datatype or parser for JSON integers
(e.g. float).

parse_constant, if specified, will be called with one of the following
strings: '-Infinity', 'Infinity', 'NaN'.
This can be used to raise an exception if invalid JSON numbers
are encountered.

Changed in version 2.7: parse_constant doesn’t get called on ‘null’, ‘true’, ‘false’ anymore.

To use a custom JSONDecoder subclass, specify it with the cls
kwarg; otherwise JSONDecoder is used. Additional keyword arguments
will be passed to the constructor of the class.

If s is a str instance and is encoded with an ASCII based encoding
other than UTF-8 (e.g. latin-1), then an appropriate encoding name must be
specified. Encodings that are not ASCII based (such as UCS-2) are not
allowed and should be decoded to unicode first.

It also understands NaN, Infinity, and -Infinity as their
corresponding float values, which is outside the JSON spec.

encoding determines the encoding used to interpret any str objects
decoded by this instance (UTF-8 by default). It has no effect when decoding
unicode objects.

Note that currently only encodings that are a superset of ASCII work, strings
of other encodings should be passed in as unicode.

object_hook, if specified, will be called with the result of every JSON
object decoded and its return value will be used in place of the given
dict. This can be used to provide custom deserializations (e.g. to
support JSON-RPC class hinting).

object_pairs_hook, if specified will be called with the result of every
JSON object decoded with an ordered list of pairs. The return value of
object_pairs_hook will be used instead of the dict. This
feature can be used to implement custom decoders that rely on the order
that the key and value pairs are decoded (for example,
collections.OrderedDict() will remember the order of insertion). If
object_hook is also defined, the object_pairs_hook takes priority.

Changed in version 2.7: Added support for object_pairs_hook.

parse_float, if specified, will be called with the string of every JSON
float to be decoded. By default, this is equivalent to float(num_str).
This can be used to use another datatype or parser for JSON floats
(e.g. decimal.Decimal).

parse_int, if specified, will be called with the string of every JSON int
to be decoded. By default, this is equivalent to int(num_str). This can
be used to use another datatype or parser for JSON integers
(e.g. float).

parse_constant, if specified, will be called with one of the following
strings: '-Infinity', 'Infinity', 'NaN'.
This can be used to raise an exception if invalid JSON numbers
are encountered.

If strict is false (True is the default), then control characters
will be allowed inside strings. Control characters in this context are
those with character codes in the 0–31 range, including '\t' (tab),
'\n', '\r' and '\0'.

If the data being deserialized is not a valid JSON document, a
ValueError will be raised.

To extend this to recognize other objects, subclass and implement a
default() method with another method that returns a serializable object
for o if possible, otherwise it should call the superclass implementation
(to raise TypeError).

If skipkeys is false (the default), then it is a TypeError to
attempt encoding of keys that are not str, int, long, float or None. If
skipkeys is true, such items are simply skipped.

If ensure_ascii is true (the default), all non-ASCII characters in the
output are escaped with \uXXXX sequences, and the results are
str instances consisting of ASCII characters only. If
ensure_ascii is false, a result may be a unicode
instance. This usually happens if the input contains unicode strings or the
encoding parameter is used.

If check_circular is true (the default), then lists, dicts, and custom
encoded objects will be checked for circular references during encoding to
prevent an infinite recursion (which would cause an OverflowError).
Otherwise, no such check takes place.

If allow_nan is true (the default), then NaN, Infinity, and
-Infinity will be encoded as such. This behavior is not JSON
specification compliant, but is consistent with most JavaScript based
encoders and decoders. Otherwise, it will be a ValueError to encode
such floats.

If sort_keys is true (default: False), then the output of dictionaries
will be sorted by key; this is useful for regression tests to ensure that
JSON serializations can be compared on a day-to-day basis.

If indent is a non-negative integer (it is None by default), then JSON
array elements and object members will be pretty-printed with that indent
level. An indent level of 0 will only insert newlines. None is the most
compact representation.

Note

Since the default item separator is ',', the output might include
trailing whitespace when indent is specified. You can use
separators=(',',':') to avoid this.

If specified, separators should be an (item_separator,key_separator)
tuple. By default, (',',':') are used. To get the most compact JSON
representation, you should specify (',',':') to eliminate whitespace.

If specified, default should be a function that gets called for objects that
can’t otherwise be serialized. It should return a JSON encodable version of
the object or raise a TypeError. If not specified, TypeError
is raised.

If encoding is not None, then all input strings will be transformed
into unicode using that encoding prior to JSON-encoding. The default is
UTF-8.

The JSON format is specified by RFC 7159 and by
ECMA-404.
This section details this module’s level of compliance with the RFC.
For simplicity, JSONEncoder and JSONDecoder subclasses, and
parameters other than those explicitly mentioned, are not considered.

This module does not comply with the RFC in a strict fashion, implementing some
extensions that are valid JavaScript but not valid JSON. In particular:

Infinite and NaN number values are accepted and output;

Repeated names within an object are accepted, and only the value of the last
name-value pair is used.

Since the RFC permits RFC-compliant parsers to accept input texts that are not
RFC-compliant, this module’s deserializer is technically RFC-compliant under
default settings.

The RFC requires that JSON be represented using either UTF-8, UTF-16, or
UTF-32, with UTF-8 being the recommended default for maximum interoperability.
Accordingly, this module uses UTF-8 as the default for its encoding parameter.

This module’s deserializer only directly works with ASCII-compatible encodings;
UTF-16, UTF-32, and other ASCII-incompatible encodings require the use of
workarounds described in the documentation for the deserializer’s encoding
parameter.

As permitted, though not required, by the RFC, this module’s serializer sets
ensure_ascii=True by default, thus escaping the output so that the resulting
strings only contain ASCII characters.

The RFC prohibits adding a byte order mark (BOM) to the start of a JSON text,
and this module’s serializer does not add a BOM to its output.
The RFC permits, but does not require, JSON deserializers to ignore an initial
BOM in their input. This module’s deserializer raises a ValueError
when an initial BOM is present.

The RFC does not explicitly forbid JSON strings which contain byte sequences
that don’t correspond to valid Unicode characters (e.g. unpaired UTF-16
surrogates), but it does note that they may cause interoperability problems.
By default, this module accepts and outputs (when present in the original
str) code points for such sequences.

The RFC does not permit the representation of infinite or NaN number values.
Despite that, by default, this module accepts and outputs Infinity,
-Infinity, and NaN as if they were valid JSON number literal values:

>>> # Neither of these calls raises an exception, but the results are not valid JSON>>> json.dumps(float('-inf'))'-Infinity'>>> json.dumps(float('nan'))'NaN'>>> # Same when deserializing>>> json.loads('-Infinity')-inf>>> json.loads('NaN')nan

In the serializer, the allow_nan parameter can be used to alter this
behavior. In the deserializer, the parse_constant parameter can be used to
alter this behavior.

The RFC specifies that the names within a JSON object should be unique, but
does not mandate how repeated names in JSON objects should be handled. By
default, this module does not raise an exception; instead, it ignores all but
the last name-value pair for a given name:

The old version of JSON specified by the obsolete RFC 4627 required that
the top-level value of a JSON text must be either a JSON object or array
(Python dict or list), and could not be a JSON null,
boolean, number, or string value. RFC 7159 removed that restriction, and
this module does not and has never implemented that restriction in either its
serializer or its deserializer.

Regardless, for maximum interoperability, you may wish to voluntarily adhere
to the restriction yourself.

This module does not impose any such limits beyond those of the relevant
Python datatypes themselves or the Python interpreter itself.

When serializing to JSON, beware any such limitations in applications that may
consume your JSON. In particular, it is common for JSON numbers to be
deserialized into IEEE 754 double precision numbers and thus subject to that
representation’s range and precision limitations. This is especially relevant
when serializing Python int values of extremely large magnitude, or
when serializing instances of “exotic” numerical types such as
decimal.Decimal.